There are many painful disorders of the spine, many relating, at least in part, to diseased or damaged intervertebral discs. Disorders include Degenerative Disc Disease, generally an age related disorder where the intervertebral disc gradually loses its water content, resiliency, and height. With a loss in intervertebral disc height and associated loss of normal spacing between vertebrae, motion of the vertebrae can place pressure on the spinal cord or exiting nerve roots. The intervertebral disc itself can also be a source of pain. Spinal disorders, commonly referred to as disc herniation and bulging disc, place painful pressure on the spinal cord and exiting nerve roots. Abnormal bone growth, called osteophytes, can place pressure on nerves or the spinal cord. Often, a surgeon must at least partially remove an intervertebral disc to access and remove an osteophyte.
A surgical approach to treating chronic spinal disorders relates to bony fusion of two adjacent vertebrae in a treatment called spine fusion. Following the achievement of appropriate spacing and alignment of the vertebral bodies, bone graft material and stabilization provide an environment for spine fusion. Implant systems, to include plate and rod systems and interbody devices, such as, interbody spacers and fusion cages can be used to support the spine during fusion. Concerns persist regarding spinal fusion treatment stemming from modest clinical success rates and the creation of rigid regions along an otherwise flexible spine.
Artificial intervertebral discs, or simply artificial discs, are an alternative to spinal fusion and represent an emerging technology. These spinal implants are designed to restore or maintain the appropriate alignment and spacing of adjacent vertebral bodies. In addition, an artificial disc is also designed for kinematic behavior similar to a healthy natural disc. Known artificial disc concepts use numerous means for providing motion and stiffness similar to a natural healthy disc, to include the adaptation of elastomers, mechanical springs, and articulating surfaces.
Prior art artificial discs often use articulating surfaces to create a joint between adjacent vertebrae. Disc implants using articulating surfaces rely on methodology and proven technology used in total joint arthroplasty of the hip, knee, and shoulder. Numerous prior art artificial discs resemble artificial hip and artificial knee joints. Numerous known artificial disc devices resemble variations of a ball-and-socket. Kuntz, in U.S. Pat. No. 4,349,921 (Sep. 21, 1982) discloses an artificial disc, with two components that articulate by means of a projection on one component pivotally engaging a depression on the second component. An artificial disc resembling an artificial knee joint has also been suggested. Shelokov, in U.S. Pat. No. 6,039,763 (Mar. 21, 2000) discloses an artificial spinal disc, similar in configuration to an artificial knee joint.
Heggeness et. al., in U.S. Pat. No. 5,514,180 (May 1996) categorizes the shape or contours of vertebral endplates into five groups: “ramp”, “saddle”, “irregular”, “bowl”, and “hump”. Heggeness et. al., teaches the importance of endplate shape relating to fit and load distribution of a prosthetic devices within intervertebral disc spaces, but Heggeness et. al. does not discuss endplate shape relating to articulating surfaces or spinal kinematics.
Spine kinematics and anatomical shapes vary by region of the spine (cervical, thoracic, and lumbar), and a need exists for artificial discs addressing specific regions of the spine, especially the unique geometry and kinematics of the cervical spine.